In vivo delivery of miRNAs for cancer therapy: challenges and strategies

Abstract

MicroRNAs (miRNAs), small non-coding RNAs, can regulate post-transcriptional gene expressions and silence a broad set of target genes. miRNAs, aberrantly expressed in cancer cells, play an important role in modulating gene expressions, thereby regulating downstream signaling pathways and affecting cancer formation and progression. Oncogenes or tumor suppressor genes regulated by miRNAs mediate cell cycle progression, metabolism, cell death, angiogenesis, metastasis and immunosuppression in cancer. Recently, miRNAs have emerged as therapeutic targets or tools and biomarkers for diagnosis and therapy monitoring in cancer. Since miRNAs can regulate multiple cancer-related genes simultaneously, using miRNAs as a therapeutic approach plays an important role in cancer therapy. However, one of the major challenges of miRNA-based cancer therapy is to achieve specific, efficient and safe systemic delivery of therapeutic miRNAs in vivo. This review discusses the key challenges to the development of the carriers for miRNA-based therapy and explores current strategies to systemically deliver miRNAs to cancer without induction of toxicity.

Keywords: Cancer therapy; Gene delivery; In vivo delivery; Nanotechnology; miRNA.

Fig. 1

Schematic representation of the microRNA generation and silencing mechanisms. Hairpin-forming pre-miRNAs are generated by pri-miRNAs, which is cleaved by Drosha. Later, pre-miRNAs are transported into the cytoplasm by exportin-5 and further converted into double-stranded mature miRNAs by Dicer. Mature miRNAs are incorporated into the RISC complex, unwound and annealed to the target mRNAs carrying complementary sequences. miRNAs are able to regulate tens to hundreds of mRNAs via the imperfect base pairing between miRNAs and the 3′ or 5′ untranslated region of the target mRNAs. The miRNA-mRNA interaction silences the target genes through mRNA cleavage or translational inhibition.

Fig. 2

Barriers of In vivo miRNA delivery for cancer therapy. The leaky structure and compression of abnormal tumor vessels lead to poor blood perfusion, which reduces the delivery efficacy of naked miRNAs. Extravascular miRNAs encounter the ECM, which blocks the penetration of miRNAs into tumors. Intravascular barriers including enzyme degradation disrupt the unmodified naked miRNAs. Also, miRNAs carried by nanoparticles larger than 100 nm in diameter increase the RES clearance in the liver, spleen, lung and bone marrow, which results in non-specific uptake by innate immune cells such as monocytes and macrophages. Moreover, miRNAs can cause immunotoxicity by triggering secretion of inflammatory cytokines through Toll-like receptors. Neurotoxicity may also be induced via miRNA-bound TLRs. Once miRNAs reach the target tumor cells, the intracellular miRNAs may be trapped in the endosomes and degraded in lysosomes. Off-target effects of miRNA may cause unwanted side effects and insufficient or saturated miRNA processing enzymes may result in deficiency of miRNAs.

Fig. 3

Strategies for miRNA delivery In vivo. Many strategies such as modified miRNA antagonists or miRNA mimics, viral vectors, inorganic or organic non-viral delivery systems have been established for delivery of miRNAs for cancer therapy. miRNA antagonists modified with LNAs bind to the targeted miRNAs with high affinity (A). Vectors encoding miRNA antagonists or miRNA mimics can be carried by viral vectors for In vivo delivery (B). Silica nanoparticles modified with GD₂ antibody specifically deliver miRNAs into tumors overexpressing GD2 (C). miRNAs can be encapsulated in organic non-viral delivery systems such as PLGA, PEI (D) and liposome based nanoparticle (E) that are modified either with ligands or with antibodies for tumor-targeted delivery (F).